1. Field of the Invention
The present invention relates to a bipolar supply voltage generator and a semiconductor device for same. More particularly, the present invention relates to a bipolar supply voltage generator which produces a positive and negative supply voltages from a unipolar power source, as well as to a semiconductor device used in that bipolar supply voltage generator.
2. Description of the Related Art
Many of today's portable data processing devices, including mobile phones and personal digital/data assistants (PDAs), have a liquid crystal display (LCD). Since LCDs use both positive and negative voltages, those devices incorporate a bipolar power supply circuit that produces such voltages from a single-voltage power source such as secondary battery cells.
FIG. 12 shows a typical configuration of a conventional bipolar supply voltage generator. The illustrated circuit comprises the following components: an input capacitor Cin, pulse generators PG1 and PG2, inductors L1 and L2, switching transistors Q1 and Q2, diodes D1 and D2, and output capacitors C1 and C2. This circuit is supplied with a source voltage Vin of, for example, three volts from a lithium secondary battery.
The input capacitor Cin is inserted between the source voltage Vin and ground to reduce the output impedance of the power source in high frequencies. The first inductor L1 is a coil with an inductance of several tens to several hundreds of microhenries (μH). The first inductor L1 stores incoming electric energy in the form of magnetic fields, and it releases that magnetic energy as electric energy. The second inductor L2 is also a coil with a similar inductance.
The pulse generators PG1 and PG2 produce a first and second pulse signals to drive two switching transistors Q1 and Q2, respectively. The switching transistor Q1 is an n-channel metal oxide semiconductor-field effect transistor (MOSFET), and Q2 is a p-channel MOSFET. The first switching transistor Q1 turns on when the first pulse signal P1 becomes high, while it is otherwise in an off state. The second switching transistor Q2, on the other hand, turns on when the second pulse signal P2 becomes low, while it is otherwise in an off state.
The two diodes D1 and D2 serve as switches that become active when their anode has a higher voltage than their cathode (forward biased), while they are otherwise shut off. The first diode D1, when forward biased, allows a voltage developed across the first inductor L1 to appear at its cathode. Likewise, the second diode D2, when forward biased, allows a voltage developed across the second inductor L2 to appear at its anode.
The first output capacitor C1 reduces output voltage ripple at the first diode D1's cathode, thus smoothing out a positive output voltage Vo1. Likewise, the second capacitor C2 reduces output voltage ripple at the second diode D2's cathode, thus smoothing out a negative output voltage Vo2.
Referring next to a timing diagram of FIG. 13, the operation of the conventional voltage generator of FIG. 12 will be described. The first pulse generator PG1 produces a first pulse signal P1 that becomes high for a predetermined period T1 at predetermined intervals as shown in part (A) of FIG. 13. The second pulse generator PG2, on the other hand, produces a second pulse signal P2 that becomes low for another predetermined period T2 at predetermined intervals as shown in part (D) of FIG. 13.
The high level of P1 makes the n-channel switching transistor Q1 turn on, which connects one end of the first inductor L1 to the ground, enabling the source voltage Vin to be fully applied to the first inductor L1. The resulting current is shown in part (C) of FIG. 13, which produces magnetic fields within the first inductor L1, where electric energy is stored in magnetic form. At this moment, however, there is no current towards the output side because the diode D1 is grounded at its anode end and thus in a back-biased condition.
The pulse signal P1 returns to the low level when a predetermined period T1 has passed after its activation. The switching transistor Q1 then turns off, and the inductor current now has to decrease. The change in the current causes self-induction of the first inductor L1, producing an electromotive force (EMF) opposing that change. Since the produced counter-EMF appears as a forward bias voltage for the first diode D1, a current path is now created from the first inductor L1 to the positive voltage output Vo1. As a result, the voltage Vo1 rises according to the decrease of the inductor current, as shown in part (B) of FIG. 13. This means that the voltage induced in the first inductor L1 pumps up the positive output voltage Vo1 through the first diode D1.
Independently of the above process, the p-channel switching transistor Q2 turns on when the second pulse signal P2 becomes low. The activated switching transistor Q2 permits the source voltage Vin to be applied to the second inductor L2, thus causing a current flowing into it as shown in part (F) of FIG. 13. The resulting current creates magnetic fields within the second inductor L2, where electric energy is stored in magnetic form. There is no current towards the output side at the moment, because the second diode D2 is biased in the backward direction.
The second pulse signal P2 returns to high when a predetermined period T2 has passed after its activation. The second switching transistor Q2 then turns off, and the inductor current now has to decrease. The change in the current causes self-induction of the second inductor L2, producing an EMF opposing that change. The produced back-EMF, a negative voltage, appears as a forward bias to the second diode D2, and therefore, a current path is created from the negative voltage output Vo2 to the second inductor L2. As a result, the magnitude of the negative voltage output Vo2 increases according to the decrease of the inductor current, as shown in part (E) of FIG. 13. This means that the voltage induced in the second inductor L2 pumps up the negative output voltage Vo2 through the second diode D2.
In the way described above, the conventional supply voltage generator produces positive and negative output voltages of about fifteen volts, out of the source voltage Vin of about three volts.
While FIGS. 12 and 13 do not illustrate it specifically, the output voltages can be regulated by using pulse frequency modulation (PFM) techniques. PFM varies the interval (or frequency) of P1 and P2 according to the actual output voltages being observed, while keeping their constant pulse widths T1 and T2. More specifically, if the actual positive output voltage Vo1 becomes higher than its nominal level, the first pulse generator PG1 will activate the first pulse signal P1 less frequently to reduce the energy transferred to the output end. If the actual voltage Vo1 is lower than its nominal level, the first pulse generator PG1 will activate P1 more frequently to raise the output voltage Vo1. The negative output voltage Vo2 can be regulated in a similar way, where the second pulse generator PG2 varies the frequency of the second pulse signal P2, depending on the actual voltage level.
As explained in FIG. 12, conventional bipolar supply voltage generators need two inductors L1 and L2 to produce two voltages with opposite polarities. Those inductors are used to store a certain amount of energy in magnetic fields produced in their core. Inductor cores are made of magnetic material, and in order to store a sufficient amount of energy, inductors have to contain a reasonable amount of magnetic material. For this reason, it is hard to reduce the physical size of inductors, unlike capacitors. Conventional bipolar supply voltage generators use at least two such space-consuming components, which makes it difficult to reduce the size of portable electronic equipment mentioned in the first part of this description.